Aqueous topcoat coating compositions, i.e., waterborne topcoat coating compositions or water-based topcoat coating compositions or waterbasecoats, are generally applied to a substrate to provide a durable, protective, and attractive cured film. One type of aqueous topcoat coating composition, a basecoat/clearcoat coating composition, may be applied in two layers. For example, a first layer formed from a pigmented basecoat coating composition may be applied to the substrate, and then a second layer formed from a clearcoat coating composition may be applied to the first layer. In contrast, another type of aqueous topcoat coating composition, a single-stage or monocoat coating composition, may be applied to the substrate in one layer.
Aqueous topcoat coating compositions generally include low volatile organic content and may be especially suitable for applications requiring cured films having metallic and/or pearlescent effects. Such aqueous topcoat coating compositions may include flake pigments to provide the cured film with the metallic and/or pearlescent effects. Metallic aluminum pigments may react with water in these aqueous topcoat coating compositions to produce hydrogen gas if the aluminum is not sufficiently passivated against corrosion. Such hydrogen gas may present a hazard for storage and handling of the aqueous topcoat coating compositions.
In addition, pearlescent pigments coated with certain metal oxides such as titanium dioxide may diminish a durability of cured films formed from aqueous topcoat coating compositions which include pearlescent pigments. For example, absorption of light by the titanium dioxide results in the production of electrons, or radicals, which may photodegrade organic binders if the metal oxides are not insulated from the organic binders in the final cured film.
The most effective treatment for preventing the gassing of aluminum flake pigments is the chromate type treatment such as that described by Kondis in U.S. Pat. No. 4,693,754, or commercially available as Hydrolux® aluminums from Eckart GmbH of Hartenstein, Germany. This chromate type treatment can prevent gassing even in the presence of ionic clay thickeners and oxidizing iron oxide pigments. However, a toxicity of chromate or vanadate requires multiple washing steps and subsequent treatment or disposal of the toxic wash materials.
Silicate encapsulation of aluminum flake pigments is described by Iler using alkalai metal silicates as in U.S. Pat. No. 2,885,366. This process uses multiple washing and filtration steps to remove inorganic salts. Failure to remove these inorganic salts results in a highly hygroscopic and water-sensitive basecoat film. After silicate encapsulation, the resulting product is a silica encapsulated aluminum flake in powder form.
There are also patents describing the use of organic coatings or organosilane coatings for passivation, such as U.S. Pat. Nos. 4,213,886 and 5,156,677. In cases which do not require filtration to produce a powdered product, corrosion-resistance in the presence of comparatively more corrosive environments is insufficient.
Another class of treatment uses organosilicates to produce a multilayer silica barrier formed from a plurality of silicate layers having low water permeability and disposed on the aluminum surface. These treatments are also able to provide adequate passivation in highly corrosive environments. Examples of this class of treatment are described in U.S. Pat. No. 9,000,068 (comparative example 18) and U.S. Pat. No. 8,900,658, and are commercially available as Hydrolan® aluminums from Eckart GmbH of Hartenstein, Germany. These plurality of silicate layers can be subsequently treated with organosilanes to improve compatibility with resinous binders. The final stage of these treatments include filtration to removes excess solvents and catalysts to provide a powdered or paste-like product. If this filtration step is omitted, then a level of organic solvent results in high basecoat volatile organic compound content and the liquid dispersion of flake pigments is not stable to settling.